It is common for molded glass containers to have a relatively thin sidewall and a relatively thicker bottom. In containers of this type, the relatively thin sidewall cools more rapidly than the bottom. It follows that this unequal cooling results in unrelieved strains remaining in the containers as they cool to room temperature; and if the unrelieved strains that remain are excessive, the container will break during the cooling process or later in storage.
In the manufacturing of molded glass containers, it has been customary to: (1) mold the containers in an individual section molding machine, (2) anneal the containers in a lehr, and then (3) cool the containers to ambient temperature.
In accordance with earlier production methods, a lehr of about 23.7 meters (78 feet) in length and a time of about 60 minutes was typically required to anneal molded glass containers within satisfactory stress levels. Moreover, such a lehr required substantial floor space and took considerable time in properly annealing containers.
In U.S. Pat. No. 3,259,481, Fuller et al., disclosed an improved method for producing glass containers in which the length of the annealing lehr was reduced to about 16.5 meters (54 feet) and the time required for glassware to progress through the lehr was reduced to about 42 minutes.
In the process as disclosed by Fuller et al., the glass containers were reduced in temperature more rapidly while the glassware remained in the upper end of the annealing range; and the temperature of the glassware was reduced more slowly after the temperature of the glassware had been reduced to the lower portion of the annealing range.
Fuller et al., utilized the well known principle that, at higher temperatures within the annealing range, a glass article has the ability to relieve strains more rapidly than at lower temperatures within a given annealing range, and at the strain point, the ability of such a glass article to reduce strain vanishes.
Thus, Fuller et al., achieved a decrease in the annealing time by simply lowering the temperature of the glassware more rapidly in the upper range of annealing temperatures where such rapid reduction would not introduce detrimental stresses into the glassware, and then lowering the temperature of the glassware more slowly in the lower range of annealing temperatures so as to avoid detrimental stresses.
As a part of their process, Fuller et al., sought to achieve uniform cooling of unequal section thicknesses of the glassware. As taught by Fuller et al., (col. 5, lines 20-23), cooling of the conveyor belt "is used to remove heat from the belt and the bottom of the ware in order that the ware temperature reduction will be substantially uniform throughout its mass."
Fuller et al., cooled the conveyor belt by providing one or more rectangularly shaped box sections upon which the conveyor belt rested. Inside each box shaped section baffles were placed that caused or allowed air to traverse back and forth inside the box section from an inlet to an outlet.
Thus, air directed through a rectangularly shaped box section cooled the box section; the cooler upper surface of the box section, upon which the conveyor belt rested, cooled the conveyor belt partially by conduction and partially by radiation; and the glassware was cooled partially by conduction between the bottom of the glassware and the conveyor and partially by radiation.
The objective of cooling the conveyor belt was to achieve uniform cooling of unequal section thicknesses and thereby to reduce the strain that is caused by unequal cooling of differing section thicknesses.
While Fuller et al., achieved a reduction in the annealing time that is required for producing glass containers, an inherent shortcoming in their apparatus and method is that the bottom of the glass containers was cooled both by conduction and by radiation with respect to the conveyor belt.
The contact of a container with a supporting surface typically is provided by a supporting rim that is disposed at, or near, the outer perimeter of the bottom of the container; and the inner portion of the bottom is recessed to provide assurance that the container is supported by the rim.
Thus, with the apparatus and method of Fuller et al., the rims of the glass containers were cooled both by conduction, as provided by contact of the rims with the conveyor belt, and by radiation of heat to the conveyor belt. In contrast, the inner portion of the bottoms were cooled by radiation alone.
Therefore, their method resulted in the bottoms of the containers being cooler near the outer perimeter of the bottoms, as opposed to the temperatures of the portions of the bottoms that were disposed radially inward of the rims.
The relatively small reduction in the annealing time as achieved by Fuller et al., is directly attributable to the fact that the outer periphery of the containers was brought down to a lower temperature than portions of the bottoms that are disposed radially inward. This fact will become apparent from the detailed description of the present invention.
In contrast, the dramatic reduction in annealing time that has been achieved by the present invention is attributable, not only to cooling the bottoms of the glass containers to lower temperatures than the sidewalls thereof, but also to lowering of a substantially centered portion of the bottoms to lower temperatures than the portions of the bottoms that lie radially outward from the cooler centered portions.